Photoelectrochemistry of Pristine Mono- and Few-Layer MoS₂

Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS₂, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS₂ structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH₃)₆]^3+/2+ and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS₂ layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS₂. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS₂ through modification of its structure, external environment, and illumination.